The present invention relates to hydroxylated polyaniline and sulfonated polyaniline electrically-conductive compositions and more particularly to their use in sensing and modulating pH of a medium in association therewith.
Polyaniline is a family of polymers that has been under intensive study recently because the electronic and optical properties of the polymers can be modified through variations of either the number of protons, the number of electrons, or both. The polyaniline polymer can occur in several general forms, including the so-called reduced form (leucoemeraldine base), possessing the general formula: ##STR1## the partially oxidized or so-called emeraldine base form, of the general formula: ##STR2## and the fully oxidized or so-called pernigraniline form, of the general formula: ##STR3##
In practice, polyaniline generally exists as a mixture of the several forms with a general formula of: ##STR4##
When 0.ltoreq.y.ltoreq.1, the polyaniline polymers are referred to as poly(paraphenyleneammeimines) in which the oxidation state of the polymer continuously increases with decreasing values of y. The fully reduced poly(paraphenyleneamine) is referred to as leucoemeraldine, having the repeating units indicated above corresponding to a value of y=1. The fully oxidized poly(paraphenyleneimine) is referred to as pernigraniline, of repeat unit shown above corresponding to a value of y=0. The partially oxidized poly(paraphenyleneamineimine), with y in the range of greater than or equal to 0.35 and less than or equal to 0.65, is termed emeraldine, though the name "emeraldine" often is focused on the compositon where y is equal to (or approximately equal to) 0.5. Thus, the terms "leucoemeraldine", "emeraldine", and "pernigraniline" refer to different oxidation states of polyaniline. Each oxidation state can exist in the form of its base or in its protonated (salt) form by treatment of the base with an acid.
The use of the terms "protonated" and "partially protonated" herein includes, but is not limited to, the addition of hydrogen ions to the polymer by, for example, a pretonic acid, such as a mineral acid and/or organic acids. The use of the terms "protonated" and "partially protonated" herein also includes pseudoprotonation, wherein a cadon such as, but not limited to, a metal ion, M.sup.+, is introduced into the polymer. For example, "50%" protonadon of emeraldine formally leads to a composition of the formula: ##STR5## which may be written as: ##STR6##
Formally, the degree of protonation may vary from a ratio of [H.sup.+ ]/[-N=]=0 to a ratio of [H.sup.+ ]/[-N=]=1. Protonation or partial protonation at the amine (--NH) sites also may occur.
The electrical and optical properties of the polyaniline polymers vary with the different oxidation states and the different forms. For example, the leucoemeraldine base, emeraldine base, and pernigraniline base forms of the polymer are electrically insulating while the emeraldine salt (protonated) form of the polymer is conductive. Protonation of emeraldine base by aqueous 1M HCl to produce the corresponding salt brings about an increase in electrical conductivity by a factor of 10.sup.12. Deprotonation occurs reversibly in aqueous base or upon exposure to vapors which form aqueous bases, such as, for example, ammonia. The emeraldine salt form also can be achieved by electrochemical oxidation of the leucoemeraldine base polymer or electrochemical reduction of the pernigraniline base polymer in the presence of an electrolyte of the appropriate pH. The rate of the electrochemical reversibility is very rapid. Solid polyaniline can be switched between conducting, protonated, and nonconducting states at a rate of approximately 10.sup.5 Hz for electrolytes in solution and even faster with solid electrolytes. (E. Paul, J. Phys. Chem., 1985, 89, 1441-1447).
The rate of electrochemical reversibility also is controlled by the thickness of the film, thin films exhibiting a faster rate than thick films. Polyaniline, then, can be reversibly switched from an insulating to a conducting form as a function of protonation level (controlled by ion insertion) and oxidation state (controlled by electrochemical potential). Thus, in contrast to, for example polypyrrole, polyaniline can be turned "on" by either an negative or a positive shift of the electrochemical potential, because polyaniline films essentially are insulating at sufficiently negative (approximately 0.00 V vs SCE) or positive (+0.7 V vs SCE) electrochemical potentials. Polyaniline also can then be turned "off" by an opposite shift of the electrochemical potential.
The conductivity of polyaniline is known to span 12 orders of magnitude and to be sensitive to pH and other chemical parameters. It is well-known that the resistance of films of both the emeraldine base and 50% protonated emeraldine hydrochloride polymer decrease by a factor of approximately 3-4 when exposed to water vapor. The resistance increases only very slowly on removing the water vapor under dynamic vacuum. The polyaniline polymer exhibits conductivities of approximately 1 to 200 Siemens per centimeter (S/cm) when approximately half of its nitrogen atoms are protonated. Electrically conductive polyaniline salts, such as fully protonated emeraldine salt [(--C.sub.6 H.sub.4 --NH--C.sub.6 H.sub.4 --NH.sup.+)--Cl-].sub.x, have high conductivity (10.sup.-4 to 10.sup.+2 S/cm) and high dielectric constants (20 to 2,000), and have a dielectric loss tangent of from below 10.sup.-3 to approximately 10.sup.1. Dielectric loss values are obtained in the prior art by, for example, carbon filled polymers, but these losses are not as large nor as readily controlled as those observed for polyaniline.
While the preparation of polyaniline polymers and the protonated derivatives thereof are known in the art, it is novel to prepare sulfonated polyaniline compositions which are capable of being "self-protonated" or "self-doped", as disclosed in the related applications cited above. Use of the terms "self-protonated" and "self-doped" herein includes, but is not limited to, the reorganization of hydrogen ions on the polymer chain. For example, self-doping or self-protonation of a polyaniline base polymer leads to a polyaniline salt polymer and a reorganization of the electronic structure which then forms a polaronic metal. The conductivity of such polaronic metal is independent of external protonation.